BAGHDAD -- When the sniper's bullet hit Billy Edwards, his Army brothers did not hesitate.

The 2nd Brigade, 3rd Infantry Division calls itself the "Send Me" brigade, and on Saturday, its soldiers were quick to send themselves to find the man who shot Pfc. William L. Edwards, a wide-eyed 23-year-old from Houston. They quickly identified the house where they believed the assailant was hiding and moved in, just as the sniper knew they would.

Inside the house, one soldier stepped on a pressure plate, detonating an estimated 30 pounds of explosives hidden under a stairwell. In an instant, four troops were killed; four others were injured. Edwards died later in the hospital. The sniper escaped.

The attack in Arab Jabour, southeast of Baghdad, was particularly savage, predicated on knowledge of the soldiers' sense of duty to a fallen comrade. Military commanders say the number of similar incidents -- those in which soldiers are lured into a house rigged to explode -- has risen dramatically across Iraq in recent months.

"The enemy is continually evolving tactics," said Maj. Gen. Rick Lynch, commander of the U.S. offensive south of Baghdad, who described Saturday's events in an interview. "In this case, our guys followed their instincts to chase this guy down and got trapped."

Saturday's attack marked the first time that troops under Lynch's command have been killed by a house-borne improvised explosive device, the official term for a house bomb. The tactic appears to have spread south from Diyala province, northeast of the capital, where three house bombs have killed several American troops in the past two months. The U.S. military typically classifies house bombs with other IED attacks, so the exact number of Americans killed by the devices is difficult to determine.

On Monday, troops in Baqubah, the capital of Diyala, held a memorial service for four soldiers killed there by a house bomb Aug. 6. Twelve other troops were killed in that attack.

In Diyala, as in Arab Jabour, the stratagem is considered a hallmark of the Sunni insurgent group al-Qaeda in Iraq, which has not asserted responsibility for Saturday's attack but wrote on an insurgent Web site that it was a cause for celebration. Though the U.S. military says the group's operations have been severely weakened across Iraq, the increased number of house bomb attacks suggest that a significant number of al-Qaeda in Iraq fighters are adapting to U.S. strategies and developing more advanced tactics of their own.

The growing use of house bombs is part of a larger pattern of more complex and coordinated attacks against U.S. forces by al-Qaeda in Iraq. On May 12, soldiers in two parked Humvees were struck by a roadside bomb near Mahmudiyah, south of Baghdad, then ambushed by gunmen in a synchronized attack that the group said it had staged. Four soldiers and an Iraqi interpreter were killed in the ambush and three soldiers were kidnapped. One was later found dead; two remain missing.

Such mass-casualty attacks underscore the vulnerability of U.S. troops fighting enemies trained in guerrilla warfare and with extensive knowledge of Iraq's often challenging terrain. In many areas south of Baghdad, where Saturday's house bomb attack and the May ambush occurred, Americans are trying to win control from native Shiite and Sunni insurgent groups.

Officials attribute the increasingly sophisticated attacks to desperation on the insurgents' part after troops became too successful at finding roadside bombs and other explosives.

"It's a clear sign that they could not get to us by other means, and that's a good sign," said Lt. Col. Michael Donnelly, a spokesman for the American operation in northern Iraq, describing the pattern of house bombs in that area. "Obviously we're countering the improvised explosive devices, and force on force, they know that they can't fight us."

But ambushes and rigged houses can cause many more casualties than smaller improvised explosive devices, which rarely kill more than one or two people at a time. Increasingly, Donnelly said, insurgents are creating a "daisy chain" of house bombs, in which an initial explosion can trigger blasts up and down a block.

Additionally, house bombs can be some of the most difficult explosives to detect because of the myriad ways they can be activated, Donnelly and others said. Some insurgents use powerful bombs or other munitions; others rely on homemade explosives. The blast can be set off by a trip wire, a pressure plate or a remote device.

"They are hard to find, but there is generally some sort of telltale sign," Donnelly said. "We just look for the signs and then deal with it the best we can."

After last week's bombing in Baqubah, troops captured three suspected insurgents, who led them to several other rigged houses in the neighborhood. Lynch said that although no one has been arrested since the Arab Jabour blast, troops have "refined their tactics" using lessons learned that day.

"We study the enemy, and we have a basic idea of how he uses these houses and how he plants initiators," Lynch said. "As their tactics evolve, ours evolve."

Donnelly said that as U.S. troops become more skilled in identifying house bombs, al-Qaeda in Iraq will probably develop even more advanced techniques for attacking soldiers. But the American military's counterinsurgency abilities, assisted by increased cooperation from Iraqi citizens, would prevail, he said.

"There is no question that there is still a serious threat," Donnelly said. "But the gains we have made are tremendous. In the end, we will win, and they will be marginalized and pushed out."